The Nervous System

Question 1

What is our nervous system made up of?

Answer 1

-CNS:brain and spinal cord
-PNS:motor neurones, sensory neurones and receptor

Question 2

Label the motor neurone

Answer 2

See notes
Should include
-axon terminals/synaptic bulbs
-cell body/ soma
-nucleus
-myelin sheaths
-nodes of Ranvier
-axon
-dendrites

Question 3

Label the sensory neurone

Answer 3

See notes
Should include
-receptor cell
-myelin sheath
-axon
-cell body

Question 4

What do we mean by the resting potential of a neurone?

Answer 4

The potential difference, normally -70mv, which is present across the resting neurone membrane, due to the fact that there is a build-up of positive Na+ ions outside the membrane

Question 5

What is depolarization?

Answer 5

The reversal of the membrane potential from -70mv to +40mv due to sodium, Na+ ions diffusing inwards

Question 6

What do we mean by the action potential?

Answer 6

The action potential is the depolarization and reversal of polarity that occurs across the neurone membrane when an impulse is being conducted

Question 7

What do we mean by repolarization of the neurone membrane?

Answer 7

This is the return to resting potential again, (+40mv to -70mv) brought about by positive ions (this time potassium) diffusing back out of the axon

Question 8

As part of repolarization, an overshoot known as hyperpolarization occurs (more negative than normal). This period is known as the REFRACTORY PERIOD. What do we mean by REFRACTORY PERIOD?

Answer 8

It is the time period during which depolarization and a further action potential cannot occur

Question 9

What is the importance of the REFRACTORY PERIOD in transmission of impulses?

Answer 9

-It ensures that impulses travel in the right direction ie onto the next neurone and not backwards
-It ensures that the AP’s are kept separate from one another ie discrete
-It limits the number of action potentials that can be fired

Question 10

What is the All or Nothing Law?

Answer 10

If a threshold level of depolarization is reached, then an Action Potential is set up and each action potential will be of the same magnitude. If the threshold isn’t reached, then there will be no AP

Question 11

How is the impulse propagated down the axon in an unmyelinated neurone?

Answer 11

Waves of depolarization travel down the axon as a series of local circuits, resulting in the resting region immediately ahead becoming depolarised, as the region immediately behind, becomes repolarised (likened to a Mexican wave)

Question 12

How is an impulse propagated in a myelinated neurone?

Answer 12

Action potentials “jump” from one Node of Ranvier to the next, due to these regions being the areas which are most permeable to the diffusion of the ions, and the fatty myelin sheath being mainly impermeable. This is known as SALTATORY conduction.

Question 13

What factors speed up electrical impulses?

Answer 13

-Myelination speeds up conduction due to saltatory conduction and the insulation prosperities of the myelin itself
-Wider acorns speed up conduction as there is less leakage of ions (making it easier to achieve resting and action potentials)
-An increase in temperature leads to faster rates of ion diffusion, therefore faster impulse conduction

Question 14

What is a Synapse

Answer 14

The junction between the end of one axon and the dendrite or cell body of the receiving neurone

Question 15

On the arrival of an AP in the synaptic bulb, which ions enter and cause synaptic vesicles to fuse with the pre-synaptic membrane?

Answer 15

Calcium ions, Ca2+

Question 16

By what process are the neurotransmitters released from the synaptic vessels and by what method do they cross the synaptic cleft/gap?

Answer 16

Exocytosis and Diffusion

Question 17

Name the neurotransmitter released by most motor neurones and say how it gets across the synaptic cleft

Answer 17

Acetylcholine- it moves via diffusion

Question 18

What happens when the neurotransmitters attach onto their complementary receptor sites on the post-synaptic membrane?

Answer 18

The sodium ion channels open and positive Na+ ions diffuse into the post-synaptic neurone

Question 19

What is an EPSP (Excitatory Post Synaptic Potential?)

Answer 19

The depolarization that occurs due to positive Na+ ions diffusing into the post-synaptic neurone. The depolarization must reach a threshold value for the action potential to be propagated onwards.

Question 20

What needs to happen to halt the firing of the action potentials in the post-synaptic neurone?

Answer 20

The neurotransmitter (often Ach, Acetylcholine) is released from the receptor sites when it is broken down by the hydrolytic enzyme, Acetylcholinesterase, into ethanoic acid and choline

Question 21

There are lots of mitochondria in the synaptic bulb. What are they needed for?

Answer 21

The mitochondria are needed to provide ATP for the active uptake of the Ach breakdown products and also to provide ATP needed to recycle Ach and package it back into the synaptic vesicles.

Question 22

What is an IPSP (inhibitory post-synaptic potential)?

Answer 22

This is the hyperpolarization that occurs in the post synaptic neurone, due to negative chloride, Cl- ions, entering the neurone. It acts to prevent AP’s being fired

Question 23

When EPSP’s and IPSP’s both occur on one post-synaptic neurone, from different neurones, what term do we use to describe what determines the outcome?

Answer 23

SUMMATION (the IPSP’s and the EPSP’s add together and whichever is “greater” determines whether an AP gets fired)

Question 24

Some insecticides work by inhibiting the enzyme acetylcholinesterase. What does this lead to?

Answer 24

Acetylcholine is not released from the receptor sites on the post-synaptic membrane, therefore continual depolarization and firing of Action Potentials occurs. The insect’s muscles are continually excited which leads to convulsions and death

Question 25

How are synapses useful?

Answer 25

-They allow one way transmission of electrical signals
-They provide integration of information from inputs that come from many neurones
-They allow fatigue to happen due to the neurotransmitter supply becoming exhausted- this prevents over-stimulation

Question 26

How do synapses ensure unidirectional impulse travel?

Answer 26

-The synaptic vesicles and neurotransmitters are only present in the pre-synaptic bulb
-The receptors for the neurotransmitters are only present on the post-synaptic membrane

Question 27

What name do we give to the special synapse that occurs between the synaptic bulk of a motor neurone and a muscle cell?

Answer 27

The neuromuscular junction

Question 28

What names do we give the
-pre-synaptic membrane
-post-synaptic membrane at the muscular junction

Answer 28

-motor end plate
-sarcolemma

Question 29

What is the potential outcome of an impulse arriving at the neuromuscular junction?

Answer 29

Muscle contraction

Question 30

If an action potential is generated at the neuromuscular junction, how does it travel through the muscle fibres?

Answer 30

Through a system of T-Tubules

Question 31

Which ions are released into the muscle fibres, ultimately leading to contraction?

Answer 31

Calcium ions